The present invention relates to measurement and detection of a leakage position or positions in an impervious or water-impermeable sheet, and in particular, to that in a impervious bottom sheet mounted in a hole or concavity formed in the land for accumulating or containing a large quantity of waste in a waste disposal site or yard.
In the waste disposal site, the impervious sheet is usually mounted in the bottom of the waste disposal concavity so as to prevent waste from flowing into the ground soil together with water. That is, the sheet is used for preventing water contaminated by waste from flowing out of the site to protect the circumference from contamination or pollution. The contaminated water would pollute the environment, for example, the ground soil and ground water.
In the waste disposal site, the impervious sheet, which is usually made of synthetic resin, rubber, or the like, is often subjected to damages, such as tear or the like, through which the contaminated water flows out or leaks out from the sheet into the ground soil. Leaked water causes a contamination of ground water and a problem of environmental pollution. Therefore, it is required to periodically inspect whether or not any leakage is caused in the impervious sheet. When any leakage position in the sheet is found out in the periodical inspection, it is necessary to repair the damage of the sheet corresponding to the leakage position.
The following conventional methods are used for detecting such a leakage position. In a first method, the impervious sheet is provided with a plurality of first electrodes made of elongated conductive materials extending in parallel with a first direction on the upper side surface or the inner side surface of the sheet, and a plurality of second electrode elements made of elongated conductive materials extending in parallel with a second direction perpendicular to the first direction on the lower side surface or the outer side surface of the sheet. A pair of upper and lower electrodes are selected one after another from the first electrodes and the second electrodes, and an electric voltage is applied across the pair of electrodes selected. Then, an electric current flowing through the pair of electrodes is measured. This is repeated for each of pairs of first and second electrodes selected one after another. When the maximum current is measured for a particular one of pairs of first and second electrodes, it is determined that the damage is located near an intersecting position of the particular one pair of first and second electrodes.
However, in the first conventional method, when there are a plurality of damages in the sheet which are apart from one another by a distance approximately equal to a space between adjacent electrode elements, the maximum current is measured for a plurality of pairs of first and second electrodes. Therefore, the conventional method has a problem that the damaged position is vague.
In another conventional method, the impervious sheet has a plurality of the first electrodes on the upper side surface thereof and a plurality of the second electrodes on the lower side surface thereof, in a similar manner as the first conventional method. By use of two electrode selective-switching circuits for the upper side and for the lower side, an AC voltage is applied to all of pairs of the first and the second electrodes one after another. A current flowing through each of the pairs of electrodes is phase-detected to form a detected output. A difference is obtained between the detected output for one pair of electrodes and that for the next subsequent pair of electrodes. When the difference is inverted in polarity between subsequent pairs, it is determined that the leakage or the damage occurs at a position between the intersections of first and second electrodes of the subsequent pairs. This method was proposed in JP-A-5 52699 by three of the present joint inventors.
In the second conventional methods, electrodes across which a test voltage is applied are also used for measuring a current. Therefore, in use of low frequency AC test voltage, an error is resulted due to an electric double layer formed on the surface of the electrode.